Type aliases are not recognized correctly in some situations with import cycles

[dfroger]

This may be a duplicate issue, but I'm a bit lost... I'm using mypy 0.600 and Python 3.6.2.

# a.py
from typing import Union

from c import T

class A:
    x: str

class B:
    x: str

U = Union[A, B]

# b.py
from c import U

def foo(u: U):
    print(u.x)

# c.py
class T:
    pass

from a import U
from b import foo

python -c 'import c' works, Mypy . give the errors:

b.py:3: error: Invalid type "a.U"
b.py:4: error: U? has no attribute "x"

I would like to understand how here Mypy import mechanism is different from Python import mechanism, and why Mypy fails with U type.

[JelleZijlstra]

Mypy doesn't exactly replicate Python's mechanism for resolving circular imports. Python can basically move back and forth between modules during execution; mypy doesn't do that.

This seems closely similar to #4930, although that one was triggered only by import *.

[dfroger]

Also maybe similar to #4442

[ghost]

Ran into a similar issue:

a.py

def incr(i: int) -> int:
    from b import Integer
    res: Integer = i + 1
    return res

b.py

from a import incr

Integer = int

Results in an error: a.py:4: error: Invalid type "Integer"

[ilevkivskyi]

I can't reproduce the original example on master. The new example still fails, but passes with the --new-semantic-analyzer flag. This should be closed when we release the version with new analyzer.

[ilevkivskyi]

Closing as this is fixed with new semantic analyzer (will be released soon).